1. Field of the Invention
The invention relates to a resist composition, which is a radiation-sensitive material used in the manufacture of semiconductor devices, and a method for forming a patterned film using the resist composition. As used herein, the radiations includes ultraviolet rays, vacuum ultraviolet rays, far-ultraviolet rays, X-rays, and electron beams.
2. Description of the Related Art
In the manufacture of semiconductor devices, such as semiconductor integrated circuits, the formation of fine patterns is indispensable. For the formation of a fine pattern on a semiconductor integrated circuit, the use of a lithography technique, in which a resist material is coated on a film provided on a substrate to be processed to thereby form a resist layer, the resist layer is then selectively exposed and developed to provide a resist pattern, the resultant resist pattern is used as a mask to dry-etch the underlying film to thereby transfer the pattern of the resist to the film, and the resist layer is then removed to leave the film having a desired pattern on the substrate, is required.
As a radiation used for the exposure of a resist layer, ultraviolet rays were initially used and, in the course of time, far-ultraviolet rays or X-rays, which have a shorter wavelength, or electron beams, have been used due to the miniaturization of the patterns to be formed. Following a change in the radiation used, a resist material, which has good resolution, high sensitivity, and excellent dry-etching resistance, corresponding to the radiation, is required. Particularly with exposure using an excimer laser beam, high energy-exposure is inevitable in order to increase the sensitivity of a resist material while retaining its resolution, due to the absorption by a benzene ring which is contained in conventional resist materials, and there have been problems, such as decreased durability of a lens made of quartz or the like used in a radiation source apparatus and the necessity of a high laser power. On the other hand, with electron beam-exposure, which inherently has a low throughput, improvement of throughput cannot be obtained unless a resist material having a high sensitivity is used.
Accordingly, different resist materials are offered corresponding to the wavelength of the light used for exposure. As one type of these materials, various chemically amplified resist materials are provided. In principle, a chemically amplified resist is a composition comprising an acid-sensitive material and an acid generator. By the use of such a composition, a resist mask having a predetermined pattern can be formed by exposing the composition to generate an acid from the acid generator under the action of light or radiation, and then reacting an acid-sensitive material using the generated acid as a catalyst by post exposure bake (post bake), to thereby cause a change in solubility of the composition in a developer. With the chemically amplified resist materials, it has been a problem to provide them with a high sensitivity without altering other properties such as resolution. This is explained hereinafter with reference to, by way of example, a chemically amplified resist used for exposure by an ArF excimer laser beam.
For instance, a resist composition using an alkaline-soluble hydrogen-added phenol resin having some of the hydroxyl groups thereof substituted with groups which are unstable in an acid is proposed in JP-A-6-51518 (Zeon Corporation). This publication provides a chemically amplified resist having balanced properties of sensitivity, resolution, etching resistance, storage stability and others by the use of the hydrogen-added phenol resin as a base resin, but does not disclose a technique for rendering the chemically amplified resist more sensitive to exposing radiation.
It is known that a resist composition using a copolymer of two methacrylic monomers, 2-methyladamantyl methacrylate and gamma-butyrolactone methacrylate, as a base resin, and also using a photo acid generator based on an onium salt of sulfonic acid and an aniline-based quencher, is a chemically amplified resist material provided with good resolution and practical dry-etching resistance. The copolymer, as the base resin in this resist material, is represented by the general formula: the copolymer incorporating 2-methyladamantyl group in place of a benzene ring, which is contained in a base resin used in a conventional resist material for ArF excimer laser beam exposure, and strongly absorbing radiation at wavelength of the ArF excimer laser beam. Such a resist material using the copolymer of two methacrylic monomers has a sensitivity of the order of 10 to 20 mJ/cm2 at conditions retaining a practical resolution, although the sensitivity depends on the process conditions used. In general, in order to ensure a practical lens life in the case of the use of an ArF excimer laser beam, it is required that a resist material can be resolved at a sensitivity of 10 mJ/cm2 or lower. The base resin in the above resist material, however, cannot have a higher sensitivity because of the use of an ester group containing a quaternary carbon, which has a high activation energy, as a group reacting upon exposure.
On the other hand, a reacting group of quaternary carbon-containing ester can provide a resist material superior in resolution, because it greatly changes its polarity before and after a reaction, and can easily yield a high contrast upon the dissolution of the material in a developer. For this reason, the quaternary carbon-containing ester group is used as a reacting group not only in resists for ArF excimer laser beam exposure but also various resists for KrF excimer laser beam, F2 laser beam, extreme ultraviolet (EUV) radiation, and electron beam exposure. However, it is also difficult for these resist materials to have both a good resolution and a high sensitivity, as the resist materials for ArF excimer laser beam exposure do.